The invention relates to improvements in methods of and in apparatus for recycling discarded catalytic converters of the type used in motor vehicles. More particularly, the invention relates to improvements in methods of and in apparatus for recycling catalytic converters of the type wherein a metallic container or housing confines normally sheet- or foil-like metallic carriers of a surface-enlarging substrate (called washcoat) for metallic catalysts.
It is known to equip motor vehicles with after-treatment devices for pollutant removal from automotive exhaust, commonly known as catalytic converters, wherein a housing or container (which can serve as a muffler) confines a ceramic body constituting a carrier of catalytic agents, particularly noble metals such as platinum, palladium and rhodium. The catalysts are effective when their temperature reaches a predetermined value. Since the mass of the ceramic body is rather large, the catalysts which are deposited thereon become effective only after a relatively long interval of time following starting of the engine in a motor vehicle. In addition, ceramic carriers are sensitive to mechanical and/or thermal shocks so that they must be installed with a view to avoid vibration and/or abrupt changes of temperature of such converters.
Attempts to overcome the drawbacks of catalytic converters which employ ceramic bodies have led to the development of aftertreatment devices wherein the catalysts are applied to metallic carriers. A catalytic converter embodying metallic carriers of catalysts employs a metallic container or housing with an inlet for exhaust gases and an outlet for treated exhaust gases. The container confines sheet- or foil-like carriers for so-called washcoats which, in turn, carry metallic catalysts. The washcoats are thin coatings which enlarge the surfaces of laminations including the carriers and the washcoats, and the enlarged surfaces carry the catalysts. As a rule, the container and its inlet and outlet are made of special high-quality steel or a nonmagnetic ferrous alloy. The sheet- or foil-like carriers are often made from a ferromagnetic Fe-Cr-A1 alloy having a minimal thickness. Flat carriers alternate with corrugated or undulate carriers, and the flat carriers contact the nodes and antinodes of corrugations forming part of the corrugated carriers. Such contacting portions of the flat and corrugated carriers can be soldered or otherwise secured to each other. The coatings of washcoat can be made of gamma-aluminum-oxide, and the exposed surfaces of such coatings are impregnated with noble metals.
An advantage of a metallic converter is that its mass is small (especially when compared with the mass of a ceramic body) so that the entire device is rapidly heated to an optimum temperature as soon as the inlet of the container begins to receive exhaust gases from the engine of a motor vehicle. In other words, the catalytic effect is felt shortly or immediately after the engine is started. Moreover, a metallic converter is less sensitive to mechanical and/or thermal stresses which, in turn, renders it possible to install the converter in the tail pipe much closer to the engine than is advisable for a ceramic converter. Of course, as the metallic converter is rather close to the engine, its temperature is raised by the exhaust gases practically immediately after starting. This, of course, is in line with the dictates and suggestions by environmental protection agencies.
In spite of their aforediscussed important advantages over ceramic converters, metallic converters have failed to find commensurate acceptance in the industry. One of the reasons is believed to be that recycling of such metallic converters is unknown or that the heretofore proposed recycling methods and apparatus are unsatisfactory.
Recycling of converters for use in motor vehicles presently involves mechanical treatment which is followed by chemical treatment serving primarily to recover the active catalysts such as platinum, palladium and rhodium. A converter is ready for recycling when its catalytic activity deteriorates. Furthermore, it is often necessary to recycle certain converters which were damaged as a result of an accident or for any other reason. In many instances, removal of spent or damaged converters from motor vehicles is not carried out with a high degree of care so that the devices which are delivered for recycling often exhibit plugged inlets and/or outlets as well as remnants of tail pipes.
Published Japanese patent application Serial No. 02-209-433 proposes to recycle a metallic catalytic converter without prior removal of the metallic container. The converter is heated in an electric furnace and is thereupon quenched in water. This is intended to result in separation of gamma-aluminum-oxide layers of the washcoats and noble metal catalysts from the foil-shaped carriers and metallic container due to different thermal expansion coefficients of the separated constituents. The mixture of gamma-aluminum-oxide and noble metals is thereupon treated with an aqueous solution of caustic soda which results in dissolution of the washcoat of gamma-aluminum-oxide. The noble metals are thereupon recovered by resorting to a filtering operation. The proposal in the published Japanese patent application merely involves segregation and an increase of the concentration of noble metals. However, the metallic container remains connected with the foil-shaped carriers and thus necessitates additional recycling. Moreover, the just discussed recycling is possible only if the damaged or spent converters are delivered with open inlet and outlet nipples.